Evaluation of a new VMAT QA device, or the “X” and “O” array geometries

Vladimir Feygelman,Craig Stevens,Geoffrey Zhang,Benjamin E Nelms

doi:10.1120/jacmp.v12i2.3346

Vladimir Feygelman, Craig Stevens + Show 2 more

Open Access

https://doi.org/10.1120/jacmp.v12i2.3346

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Abstract

We introduce a logical process of three distinct phases to begin the evaluation of a new 3D dosimetry array. The array under investigation is a hollow cylinder phantom with diode detectors fixed in a helical shell forming an “O” axial detector cross section (ArcCHECK), with comparisons drawn to a previously studied 3D array with diodes fixed in two crossing planes forming an “X” axial cross section (Delta 4 ). Phase I testing of the ArcCHECK establishes: robust relative calibration (response equalization) of the individual detectors, minor field size dependency of response not present in a 2D predecessor, and uncorrected angular response dependence in the axial plane. Phase II testing reveals vast differences between the two devices when studying fixed‐width full circle arcs. These differences are primarily due to arc discretization by the TPS that produces low passing rates for the peripheral detectors of the ArcCHECK, but high passing rates for the Delta 4 . Similar, although less pronounced, effects are seen for the test VMAT plans modeled after the AAPM TG119 report. The very different 3D detector locations of the two devices, along with the knock‐on effect of different percent normalization strategies, prove that the analysis results from the devices are distinct and noninterchangeable; they are truly measuring different things. The value of what each device measures, namely their correlation with – or ability to predict – clinically relevant errors in calculation and/or delivery of dose is the subject of future Phase III work.PACS number: 87.55Qr

Highlights

As rotational therapy grows in popularity, new quality assurance (QA) strategies are emerging, one of which is the use of 3D dosimetry phantoms to allow the entire rotational plan to be delivered to the phantom and the measured dose values compared to treatment planning system (TPS) calculations on the virtual model of the phantom
There have been previous studies on 3D dosimetry phantoms, including a high-density volumetric detector system made of solid gel,(14-18) radiographic film curved in a spiral pattern,(19) and two orthogonal planes of point detectors with an “X” axial cross section.[20,21,22,23] The purpose of this work was to perform Phase I and Phase II validation testing on a new commercial 3D dosimetry system with an “O” detector arrangement cross section called “ArcCHECK” (Sun Nuclear Corporation, Melbourne, FL)
Dosimetry of full arcs with fixed apertures Since the purpose of this paper is the evaluation of a new 3D dosimetry phantom with the emphasis on volumetric modulated arc therapy (VMAT) QA, a baseline of performance should be established by delivering simple full arcs of varying widths and covering, at some dose level, the entirety of detectors

Summary

Introduction

A. QA devices evolve with planning and delivery systems As radiation therapy becomes ever more customizable to each individual patient, the complexities of the supporting treatment planning system (TPS) and the delivery system increase. The following types of X-ray radiation therapy delivery systems are different enough from each other that they demand unique QA strategies: static gantry intensity-modulation radiation therapy (IMRT), helical TomoTherapy, volumetric modulated arc therapy (VMAT), and robotic arm therapy. 147 Feygelman et al: Evaluation of a VMAT QA device vs TPS calculation) in a single plane in a flat phantom. This strategy was summarized in detail in the AAPM TG 119 report on the IMRT commissioning.[10] this approach is less than ideal for a rotating beam.

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